[11] For were the Soil as dense as Glass, the Roots or Vegetables (such as our Earth produces) would never be able to enter its Pores. These Interstices, by their Number and Largeness, determine the specific Gravity (or true Quantity) of every Soil: The larger they are, the lighter is the Soil; and the inner Superficies is commonly the less. The Mouths, or Lacteals, being situate, and opening, in the convex Superficies of Roots, they take their _Pabulum_, being fine Particles of Earth, from the Superficies of the Pores, or Cavities, wherein the Roots are included. And ’tis certain, that the Earth is not divested or robb’d of this _Pabulum_, by any other Means, than by actual Fire, or the Roots of Plants. For, when no Vegetables are suffer’d to grow in a Soil, it will always grow richer. Plow it, harrow it, as often as you please, expose it to the Sun in Horse-Paths all the Summer, and to the Frost of the Winter; let it be cover’d by Water at the Bottom of Ponds, or Ditches; or if you grind dry Earth to Powder, the longer ’tis kept exposed, or treated by these or any other Method possible (except actual Burning by Fire); instead of losing, it will gain the more Fertility. These Particles, which are the _Pabulum_ of Plants, are so very minute[12] and light, as not to be singly attracted to the Earth, if separated from those Parts to which they adhere[13], or with which they are in Contact (like Dust to a Looking-Glass, turn it upwards, or downwards, it will remain affixt to it), as these Particles do to those Parts, until from thence remov’d by some Agent. [12] As to the Fineness of the _Pabulum_ of Plants, ’tis not unlikely, that Roots may insume no grosser Particles, than those on which the Colours of Bodies depend; but to discover the greatest of those Corpuscles, Sir _Isaac Newton_ thinks, it will require a Microscope, that with sufficient Distinctness can represent Objects Five or Six hundred times bigger, than at a Foot Distance they appear to the naked Eye. My Microscope indeed is but a very ordinary one, and when I view with it the Liquor newly imbibed by a fibrous Root of a Mint, it seems more limpid than the clearest common Water, nothing at all appearing in it. [13] Either Roots must insume the Earth, that is their _Pabulum_, as they find it in whole Pieces, having intire Superficies of their own, or else such Particles as have not intire Superficies of their own, but want some Part of it, which adheres to, or is Part of the Superficies of larger Particles, before they are separated by Roots. The former they cannot insume (unless contained in Water); because they would fly away at the first Pores that were open: _Ergo_ they must insume the latter. A Plant cannot separate these Particles from the Parts to which they adhere, without the Assistance of Water, which helps to loosen them.